Alpha channel and specular map together
Maybe you have an object just for the label, in this case use an 'add' shader to combine your diffuse and glossy shaders and connect them to the mix.

If you had only one object and your specular was supposed to be applied to metal parts only, then you could use a mix with the inverted alpha as factor. 

You can mix and add shaders without limits so setting up these kind of material is preferable in some cases (if you wanted to add a texture that affects both the can and the label).

This is what I mean: